Multi-refrigerant cooling system with provisions for adjustment of refrigerant composition

ABSTRACT

A multi-refrigerant cooling system comprising a cooling circuit ( 100 ) for circulation of a refrigerant mixture. The cooling circuit comprises, i.a., one or more separator(s) ( 102, 103 ) configured to separate and withdraw a respective refrigerant fraction of the refrigerant mixture. Each separator is connected to a respective holding tank ( 201, 202 ). Each holding tank is further connected to the cooling circuit via a supply conduit ( 207 ), the supply conduit being configured to supply one or more refrigerant fraction(s) to the cooling circuit. Method for adjusting the composition of a refrigerant mixture of a multi-refrigerant cooling system. The method allows adjustment of the composition of the refrigerant mixture during operation of the multi-refrigerant cooling system.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the benefit of U.S. Provisional Patent Application Ser. No. 61/058,947 filed Jun. 5, 2008, which is hereby incorporated by reference in its entirety.

FIELD OF THE INVENTION

The present invention is directed to a multi-refrigerant cooling system as well as to a method for adjusting the composition of a refrigerant mixture of a multi-refrigerant cooling system.

BACKGROUND

Multi-refrigerant cooling systems are previously known and operate with a refrigerant mixture of two or more refrigerants having different condensation temperatures. Thus, a mixture of refrigerants is circulated in the cooling circuit of the multi-refrigerant cooling system. Multi-refrigerant cooling systems are used, in particular, in industrial applications demanding very low temperatures. A typical application is the capture of carbon dioxide (CO₂) from exhaust gases by frosting of CO₂ ice.

Multi-refrigerant cooling systems are disclosed in for example U.S. Pat. No. 7,073,348 and US 2006/0277942. The disclosed systems operate according to a cooling principle called integrated cascade.

In order to change the composition of the refrigerant mixture, a multi-refrigerant cooling system is shut off, emptied of its refrigerant mixture and refilled with a refrigerant mixture of desired composition.

SUMMARY

Objects of the present invention include the provision of a possibility for re-use of refrigerants when an adjustment of the composition of a refrigerant mixture of a multi-refrigerant cooling system is performed; the provision of a possibility of keeping the amount of refrigerants that is wasted during an adjustment of the composition of a refrigerant mixture of a multi-refrigerant cooling system to a minimum; and the provision of a possibility to carry out an adjustment of the composition of the refrigerant mixture during maintained operation of a multi-refrigerant cooling system.

It is of economical as well as environmental interest to avoid or reduce discharge of refrigerants from a multi-refrigerant cooling system in connection with adjustment of a composition of a refrigerant mixture. It is furthermore of operational as well as economical interest to allow adjustment of the composition of the refrigerant mixture during operation of the multi-refrigerant cooling system.

The above-mentioned objects as well as further objects, which will become apparent to a skilled person after studying the description below, are achieved, in a first aspect, by a multi-refrigerant cooling system comprising a cooling circuit for circulation of a refrigerant mixture comprising two or more refrigerants, the cooling circuit comprising a compressor having an inlet and an outlet; one or more separator(s) configured to separate and withdraw a respective refrigerant fraction of the refrigerant mixture; and a client, the outlet of the compressor being connected to the client via the separator(s), wherein each separator is connected to a respective holding tank via a respective withdrawal conduit, each holding tank being arranged to receive said respective refrigerant fraction from its respective separator, wherein each holding tank further is connected to the cooling circuit via a supply conduit, the supply conduit being configured to supply one or more refrigerant fraction(s) to the cooling circuit.

Thus, it is provided a multi-refrigerant cooling system allowing adjustment of the composition of its refrigerant mixture to be performed under favourable conditions. In particular, adjustment in view of changing client temperature requirements and or changing environmental temperature can be addressed by changing the composition of the refrigerant blend during operation of the cooling system.

As used herein, the term “client” relates to an item, along the cooling circuit, which is to be cooled by the multi-refrigerant cooling system. Apart from what is described herein, the detailed layout of the cooling circuit, or its working principle, is not critical to the present invention.

The system may comprise a further holding tank connected via a further withdrawal conduit to the cooling circuit at a position between the separator(s) and the client, the further holding tank being arranged to receive a refrigerant fraction from the cooling circuit, wherein the further holding tank further is connected to the cooling circuit via the supply conduit. Thus, also a refrigerant fraction remaining in the cooling circuit after separation, and subsequent withdrawal to holding tank(s), of one or more other refrigerant fraction(s) by the separator(s), may be utilized when adjusting the composition of the refrigerant mixture.

The supply conduit may be connected to the cooling circuit at a position between the client and the inlet of the compressor. A refrigerant fraction is typically maintained in its holding tank at the pressure of its respective separator or slightly below. Since the separator(s) typically belong to the high pressure side of the cooling circuit, it is beneficial to supply such a refrigerant fraction from its holding tank via the supply conduit to the cooling circuit at a position between the client and the inlet of the compressor, i.e. on the low pressure side of the cooling circuit. Thus, it is possible to supply to the cooling circuit such a refrigerant fraction without, or with less need for, pumps or other pressure regulating means. Therefore, the system may be provided with holding tank(s) that is/are maintained at a pressure between the pressure in the cooling circuit where the respective refrigerant fraction is separated and the pressure in the cooling circuit where the supply conduit is connected.

Each withdrawal conduit may further be connected to a flare. If a withdrawn fraction is not of desirable purity, this fraction may be removed from the cooling circuit, rather than stored and/or reused. Therefore, a separated and withdrawn refrigerant fraction of the refrigerant mixture may be discarded. This may conveniently be achieved by passing of the separated and withdrawn refrigerant fraction to an outlet flare.

As an example, the client may be a carbon dioxide frosting vessel, i.e. a vessel in which gaseous carbon dioxide is captured as carbon dioxide ice at low temperature. Accordingly, the present invention additionally relates to the use of a multi-refrigerant cooling system as described above for cooling of a carbon dioxide frosting vessel.

The system may be configured to be controlled by a control system. A control device with associated control signalling infrastructure keep track of the quantities in each holding tank via, for example pressure sensors, and the control device may further keep track of the percentages of each refrigerant in the system via a multi-component detector. The control device may also determine the rate and length of opening of different control valves, according to what adjustment to the refrigerant mixture is needed.

In a second aspect, certain objects of the present invention are accomplished by a method for adjusting the composition of a refrigerant mixture of a multi-refrigerant cooling system, said method comprising the following steps:

a) withdrawing from the multi-refrigerant cooling system one or more fraction(s) of the refrigerant mixture, said fractions being of different refrigerant compositions;

b) supplying to the multi-refrigerant cooling system a refrigerant stream;

so that the composition of the refrigerant mixture of the multi-refrigerant cooling system is adjusted to a new composition, said new composition being different from the composition of the refrigerant stream; and

c) maintaining during steps a) and b) the refrigerant mixture of the multi-refrigerant cooling system in an amount allowing operation of the multi-refrigerant cooling system;

thereby allowing adjustment of the composition of the refrigerant mixture during operation of the multi-refrigerant cooling system.

It is of economical as well as environmental interest to avoid or reduce discharge of refrigerants from the multi-refrigerant cooling system in connection with adjustment of the composition of a refrigerant mixture. Thus, one or more, preferably all, of the fraction(s) withdrawn in step a) may be individually stored. Storing the withdrawn fractions individually facilitates further treatment, such as recovery or recycling, thereof. Any fraction not stored may be discarded, e.g., flared off. A withdrawn fraction may typically be discarded, rather than stored, if this fraction is not of desirable purity.

In order not only to avoid or reduce discharge of refrigerants but also to achieve a desirable closing of the operation of a multi-refrigerant cooling process, the refrigerant stream supplied in step b) may comprise, preferably consist of, one or more of the stored fraction(s). Fraction(s) withdrawn from the multi-refrigerant cooling system may thus be returned thereto, albeit in amounts and/or proportions rendering the composition of the refrigerant mixture of the multi-refrigerant cooling system to change. Make-up refrigerants, not withdrawn from the multi-refrigerant cooling system but typically supplied substantially pure or in mixtures of a set composition, may conveniently be supplied to the multi-refrigerant cooling system as part of the refrigerant stream of step b).

Typically, operation of a multi-refrigerant cooling system inherently involves separation of its refrigerant mixture into different fractions corresponding at large to the respective refrigerants of the refrigerant mixture. Thus, beneficially the number of fractions withdrawn in step a) may be equal to or less than, preferably equal to, the number of refrigerants in the refrigerant mixture of the multi-refrigerant cooling system.

Occasionally, it may not be useful to store any withdrawn fraction of the refrigerant mixture, i.e., the fraction(s) withdrawn in step a) may be discarded. As mentioned above, such fraction(s) may be flared off. In such a situation, the number of fractions withdrawn in step a) may suitably be one. Make-up refrigerants, not withdrawn from the multi-refrigerant cooling system but typically supplied substantially pure or in mixtures of a set composition, may however be supplied to the multi-refrigerant cooling system as part of the refrigerant stream of step b).

The stored fraction(s) may each be maintained at a pressure between the pressure of the refrigerant mixture at the position of the multi-refrigerant cooling system where in step a) the respective fraction is withdrawn and the pressure of the refrigerant mixture at the position of the multi-refrigerant cooling where step b) is performed. Alternatively or additionally, the fraction(s) withdrawn in step a) may each be withdrawn at a position of the multi-refrigerant cooling system where the refrigerant mixture is present at a higher pressure than at the position of the multi-refrigerant cooling system where step b) is performed. As a favourable consequence of considering pressure aspects as suggested here, withdrawal of fraction(s) of refrigerant mixture, storing of such fraction(s) and/or supply of the refrigerant stream can be performed without, or with less need for, pumps or other pressure regulating means.

As an example, the multi-refrigerant cooling system may cool a carbon dioxide frosting vessel, i.e. a vessel in which gaseous carbon dioxide is captured as carbon dioxide ice at low temperature.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustration of a multi-refrigerant cooling system according to an embodiment of the invention.

FIG. 2 is a schematic illustration of another multi-refrigerant cooling system according to an embodiment of the invention.

DETAILED DESCRIPTION

In FIG. 1 is depicted a multi-refrigerant cooling system comprising a cooling circuit 100 for circulation of a refrigerant mixture and arrangements 200 for adjusting the composition of the refrigerant mixture.

The cooling circuit 100 comprises a compressor 101, refrigerant separators 102 and 103 as well as a client 104 which is to be cooled by the multi-refrigerant cooling system. The outlet of the compressor 101 is connected to the client 104 via the separators 102 and 103. The cooling circuit 100 further comprises expansion means 105, 106 and 107 for different fractions of the refrigerant mixture as well as heat exchangers (condensers/evaporators) 108 and 109, all laid-out for operation of the cooling circuit according to a cooling principle called integrated cascade. The illustrated cooling circuit is designed for a refrigerant mixture of three components. It is emphasized that the detailed layout of the cooling circuit, or its working principle, is not critical to the present invention.

The arrangements 200 for adjusting the composition of the refrigerant mixture comprise holding tanks 201, 202 and 203 for each of the refrigerants making up the refrigerant mixture being circulated in the cooling circuit 100. The holding tanks are connected to the cooling circuit 100 via respective valves 204, 205 and 206 and a supply conduit 207. The supply conduit 207 is connected to the cooling circuit 100 at a position between the client 104 and the inlet of the compressor 101, i.e. on the low pressure side of the cooling circuit. A flare 208 is connected via a valve 209 to the cooling circuit 100.

Not shown in FIG. 1 is a control device with associated control signalling infrastructure which keeps track of the quantities in each holding tank 201, 202 and 203 via pressure sensors, and of the percentages of each refrigerant in the refrigerant mixture of the cooling circuit 100 via a multi-component detector, and controls valves 204, 205, 206 and 209.

To adjust the composition of the refrigerant mixture of the cooling circuit 100, the cooling circuit is emptied of a portion of the refrigerant mixture and re-filled with suitable amounts of one or more refrigerants in order to influence the composition of the mixture. Thus, the valve 209 is temporarily opened to allow a portion of the refrigerant mixture to be vented from the cooling circuit 100 to the flare 208. One or more of the valves 204, 205 and 206 are temporarily opened to allow refrigerant(s) from holding tanks 201, 202 and/or 203 to be supplied to the cooling circuit 100 via the supply conduit 207. The control device (not shown) determines the rate and length of opening of the valves according to what adjustment is to be made and outputs corresponding signals to one or more of the valves 204, 205, 206 and 209 as may be required.

In FIG. 2 is depicted another multi-refrigerant cooling system comprising a cooling circuit 100 for circulation of a refrigerant mixture and arrangements 200 for adjusting the composition of the refrigerant mixture.

The cooling circuit 100 in FIG. 2 is similar to the cooling circuit 100 of FIG. 1. It is, however, again emphasized that the detailed layout of the cooling circuit, or its working principle, is not critical to the present invention.

The arrangements 200 for adjusting the composition of the refrigerant mixture comprise holding tanks 201 and 202, each connected to a respective separator 102 or 103 via respective withdrawal conduits 210 and 211 and respective valves 212 and 213. A further holding tank 203 is connected via a withdrawal conduit 214 and a valve 215 to the cooling circuit 100 at a position between the separator 103 and the client 104. Each holding tank 201, 202 and 203 is thus arranged to receive a respective refrigerant fraction from the cooling circuit 100. The holding tanks are connected to the cooling circuit 100 via respective valves 204, 205 and 206 and a supply conduit 207. The supply conduit 207 is connected to the cooling circuit 100 at a position between the client 104 and the inlet of the compressor 101, i.e. on the low pressure side of the cooling circuit. A flare 208 is connected via respective valves 216, 217 and 218 to the withdrawal conduits 210, 211 and 214.

Not shown in FIG. 2 is a control device with associated control signalling infrastructure which keeps track of the quantities in each holding tank 201, 202 and 203 via pressure sensors, and of the percentages of each refrigerant in the refrigerant mixture of the cooling circuit 100 as well as of the percentages of each refrigerant in the refrigerant fractions in each holding tank 201, 202 and 203 via a multi-component detector, and controls valves 204, 205, 206, 212, 213, 215, 216, 217 and 218. The control device may include, for example, a general-purpose computer, application specific computing device or other programmable controller that receives input signals indicative of these system parameters, processes the input signals using stored instructions, and provides output signals to the various control valves to operate the system in the manner described herein.

To adjust the composition of the refrigerant mixture of the cooling circuit 100, the cooling circuit is emptied of a portion of the refrigerant mixture and re-filled with suitable amounts of one or more refrigerant fractions in order to influence the composition of the mixture. Thus, one or more of the valves 212, 213 and 215 are temporarily opened to allow a portion of the respective refrigerant fractions to pass to the respective holding tanks 201, 202 or 203, or is one or more of the valves 216, 217 and 218 temporarily opened to allow a portion of the respective refrigerant fraction to be vented from the cooling circuit 100 to the flare 208. One or more of the valves 204, 205 and 206 are temporarily opened to allow refrigerant fractions(s) from holding tanks 201, 202 and/or 203 to be supplied to the cooling circuit 100 via the supply conduit 207. The control device (not shown) determines the rate and length of opening of the valves according to what adjustment is to be made and outputs corresponding signals to one or more of the valves 204, 205, 206, 212, 213, 215, 216, 217 and 218 as may be required.

The multi-refrigerant cooling system of FIG. 2 may alternatively be described as follows.

The multi-refrigerant cooling system (MRC) comprises a cooling circuit 100 for circulation of a refrigerant mixture comprising two or more refrigerant fractions. The MRC operates by blending two or more refrigerants with different condensation temperatures in one process. The cooling circuit 100 comprises a compressor 101, a client 104, one or more separator(s) 102, 103 located between the compressor 101 and the client 104 in the circuit 100. Each separator 102, 103 is configured to being able to, in addition to separate, also withdraw a particular refrigerant fraction through a piping connector 210, 211 from the lower part of each separator of the refrigerant mixture, where the piping connector(s) 210, 211 is/are connected to a particular holding tank 201, 202 that holds one particular refrigerant. The holding tank(s) 201, 202 is/are arranged to receive a particular fraction from its respective separator 102, 103. Each of the piping connectors 210, 211 is equipped with two sets of control valves 212, 216; 213, 217, wherein one set 212, 213 regulates flow into the holding tank and one set 216, 217 regulates flow to an outlet flare 208. Each holding tank 201, 202 is further and separately connected through a pipe wherein each said pipe is joined to the cooling circuit through common header pipe 207. Each of the pipes is fitted with a control valve 204, 205 which is configured to regulate the supply of one or more particular refrigerant fraction(s) from one or more of the holding tank(s) 201, 202 to the cooling circuit 100. The refrigerant with the lowest condensation temperature does not have its own separator and exists in a pure state after the last separator 103, and is connected to its holding tank 203 through a piping connection 214. This piping connection 214 is equipped with one set of control valves 215, 218, wherein one 215 regulates flow into the holding tank 203 and one 218 regulates flow to an outlet flare 208. The holding tank 203 is also separately connected through a pipe to the cooling circuit through common header pipe 207. This pipe is also fitted with a control valve 206 which is configured to regulate the supply of the particular refrigerant fraction from the holding tank 203 to the cooling circuit 100.

To lower the quantity of a particular refrigerant in the MRC system, the respective control valve 212, 213, 215 is opened to allow the desired refrigerant to exit the MRC and to either enter its respective holding tank 201, 202, 203 or to be vented to the flare by opening one or more of the control valves 216, 217, 218. To increase the quantity of a particular refrigerant in the MRC system, the respective control valve 204, 205, 206 is opened, to allow the respective refrigerant to exit its respective holding tank 201, 202, 203 and to enter the process flow, preferably on the low pressure side. These refrigerant transfers may be achieved by using the differential pressures only, without the need for pumping.

While there have been described what are presently considered to be preferred embodiments, it will be understood by those skilled in the art that other modifications can be made within the spirit of the invention. The above descriptions of embodiments are not intended to be exhaustive or limiting in scope. It should be understood that the invention is not limited to the embodiments described above, but rather should be interpreted within the full meaning and scope of the appended claims. 

1. A multi-refrigerant cooling system comprising a cooling circuit (100) for circulation of a refrigerant mixture comprising two or more refrigerants, the cooling circuit comprising a compressor (101) having an inlet and an outlet; one or more separator(s) (102, 103) configured to separate and withdraw a respective refrigerant fraction of the refrigerant mixture; and a client (104), the outlet of the compressor (101) being connected to the client (104) via the separator(s) (102, 103), wherein each separator (102, 103) is connected to a respective holding tank (201, 202) via a respective withdrawal conduit (210, 211), each holding tank (201, 202) being arranged to receive said respective refrigerant fraction from its respective separator (102, 103), wherein each holding tank (201, 202) further is connected to the cooling circuit (100) via a supply conduit (207), the supply conduit (207) being configured to supply one or more refrigerant fraction(s) to the cooling circuit (100).
 2. The system according to claim 1, comprising a further holding tank (203) connected via a further withdrawal conduit (214) to the cooling circuit (100) at a position between the separator(s) (102, 103) and the client (104), the further holding tank (203) being arranged to receive a refrigerant fraction from the cooling circuit (100), wherein the further holding tank (203) further is connected to the cooling circuit (100) via the supply conduit (207).
 3. The system according to claim 1, wherein the supply conduit (207) is connected to the cooling circuit (100) at a position between the client (104) and the inlet of the compressor (101).
 4. The system according to claim 1, wherein each withdrawal conduit (210, 211, 214) is further connected to a flare (208).
 5. The system according to claim 1, wherein the client is a carbon dioxide frosting vessel.
 6. Method for adjusting the composition of a refrigerant mixture of a multi-refrigerant cooling system, said method comprising the following steps: a) withdrawing from the multi-refrigerant cooling system one or more fraction(s) of the refrigerant mixture, said fractions being of different refrigerant compositions; b) supplying to the multi-refrigerant cooling system a refrigerant stream; so that the composition of the refrigerant mixture of the multi-refrigerant cooling system is adjusted to a new composition, said new composition being different from the composition of the refrigerant stream; and c) maintaining during steps a) and b) the refrigerant mixture of the multi-refrigerant cooling system in an amount allowing operation of the multi-refrigerant cooling system; thereby allowing adjustment of the composition of the refrigerant mixture during operation of the multi-refrigerant cooling system.
 7. Method according to claim 6, wherein one or more, preferably all, of the fraction(s) withdrawn in step a) is/are individually stored.
 8. Method according to claim 7, wherein the refrigerant stream supplied in step b) comprises, preferably consists of, one or more of the stored fraction(s).
 9. Method according to claim 7, wherein the number of fractions withdrawn in step a) is equal to or less than, preferably equal to, the number of refrigerants in the refrigerant mixture of the multi-refrigerant cooling system.
 10. Method according to claim 6, wherein the fraction(s) withdrawn in step a) is/are discarded.
 11. Method according to claim 10, wherein the number of fractions withdrawn in step a) is one.
 12. Method according to claim 7, wherein the stored fraction(s) is/are each maintained at a pressure between the pressure of the refrigerant mixture at the position of the multi-refrigerant cooling system where in step a) the respective fraction is withdrawn and the pressure of the refrigerant mixture at the position of the multi-refrigerant cooling where step b) is performed.
 13. Method according to claim 6, wherein the fraction(s) withdrawn in step a) is/are each withdrawn at a position of the multi-refrigerant cooling system where the refrigerant mixture is present at a higher pressure than at the position of the multi-refrigerant cooling system where step b) is performed.
 14. Method according to claim 6, wherein the multi-refrigerant cooling system cools a carbon dioxide frosting vessel. 